450 Dr. W. C. Roberts-Austen. 



plate need only be exposed to the action of the spot of light at the 

 critical moment, when solidification or surfusion is known to be 

 imminent. A paper by my assistant, Mr. Stansfield, illustrating 

 the use of this method, will shortly be published by the Physical 

 Society. 



A curve, traced by the aid of either of the sensitive methods 

 which have just been described, if it represents the surfusion of a 

 metal or alloy does not merely show a slight depression as in the 

 case of pure gold shown at a, fig. 3 : the slight depression becomes a 

 deep dip, (Plate 8, fig. 1, which represents the surfusion of gold). 

 In this case three curves were taken on one plate, the line ab 

 represents the heating and melting of gold, the horizontal portion 

 marking the actual melting of the metal. The lines cde and fgh, 

 on the other hand, represent successive coolings and solidifications 

 of the metal, surfusion occurring in both cases, the horizontal por- 

 tions at e and h representing the beginning of solidification of the 

 gold in each case. It is noteworthy that these successive points 

 of solidification differ by less than half a degree, the melting point of 

 the metal (the line ab) occupying a median position between them. 

 During the surfusion, the temperature of the metal fell about 2 

 below its true freezing point. Pigs. 2 and 3, Plate 8, both represent 

 the surfusion of gold ; fig. 4, Plate 8, that of copper,* while fig. 5, 

 Plate 8, represents the surfusion of an alloy of antimony with 25 per 

 cent, of copper which freezes at 520. It was selected as represent- 

 ing the highest temperature of surfusion which I have as yet 

 examined in the case of an alloy. Such a surfusion curve, in the 

 case of an alloy, may do much more than reveal the sudden release 

 of the latent heat of the fluid mass. The curve may present a com- 

 plicated record of modifications in molecular grouping occurring in 

 the brief space of time actually occupied by surfusion. This is 

 shown by the points d, e, in fig. 6, Plate 9, which represents the sur- 

 fusion of tin alloyed with 36'5 per cent, of lead. All the curves on 

 Plate 9 serve as bonds connecting the behaviour of alloys with that 

 of freezing solutions of salts. 



The explanation of the existence of these points, d, e, is simple. 

 The freezing point curve of the lead-tin series is a very ordinary 

 type, and is shown in the accompanying diagram (fig. 4), and, as 

 regards the portion where the lines meet, full details are given on a 

 larger scale in fig. 5. It consists of two branches united at the point 

 where they join a horizontal line which represents the freezing of the 



* Figs. 2, 3, 4 on Plate 8 were taken with an insensitive galvanometer and have 

 been enlarged by photography from the original plates. Fig. 5, Plate 8, and all the 

 curves on Plate 9 are given exactly as they were taken on the plate of the 

 recording instrument. The co-ordinates, as in the case of the figures 1 to 4, are 

 time and temperature. 



